There has been a rapid growth in the market for linear low density polyethylene (LLDPE), particularly resin made under mild operating conditions; typically at pressures of 100 to 400 psi and reaction temperatures of less than 120.degree. C. This low pressure process provides a broad range of LLDPE products for blown and cast film, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. LLDPE has essentially a linear backbone with only short chain branches, about 2 to 6 carbon atoms in length. In LLDPE, the length and frequency of branching, and, consequently, the density, is controlled by the type and amount of comonomer used in the polymerization. Although the majority of the LLDPE resins on the market today have a narrow molecular weight distribution, LLDPE resins with a broad molecular weight distribution are available for a number of non-film applications.
LLDPE resins designed for commodity type applications typically incorporate 1-butene as the comonomer. The use of a higher molecular weight alpha-olefin comonomer produces resins with significant strength advantages relative to those of ethylene/1-butene copolymers. The predominant higher alpha-olefin comonomers in commercial use are 1-hexene, 4-methyl-1-pentene, and 1-octene. The bulk of the LLDPE is used in film products where the excellent physical properties and drawdown characteristics of LLDPE film makes this film well suited for a broad spectrum of applications. Fabrication of LLDPE film is generally effected by the blown film and slot casting processes. The resulting film is characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance.
These properties together with toughness are enhanced when the polyethylene is of high molecular weight. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases. By providing a blend of polymers, the properties characteristic of high molecular weight resins can be retained and processability, particularly the extrudability (from the lower molecular weight component) can be improved.
The blending of these polymers is successfully achieved in a staged reactor process similar to those described in U.S. Pat. No. 5,047,468, 5,149,738 and 5,665,818. Briefly, the process is one for the in situ blending of polymers wherein a higher density ethylene copolymer is prepared in a high melt index reactor and a lower density ethylene copolymer is prepared in a low melt index reactor. The process typically comprises continuously contacting, under polymerization conditions, a mixture of ethylene and one or more alpha-olefins with a catalyst system in two gas phase, fluidized bed reactors connected in series, said catalyst system comprising: (i) a supported magnesium/titanium based catalyst precursor; (ii) an aluminum containing activator compound; and (iii) a hydrocarbyl aluminum cocatalyst, the polymerization conditions being such that an ethylene copolymer having a melt index in the range of about 0.1 to about 1000 grams per 10 minutes is formed in the high melt index reactor and an ethylene copolymer having a melt index in the range of about 0.001 to about 1 gram per 10 minutes is formed in the low melt index reactor, each copolymer having a density of about 0.860 to about 0.965 gram per cubic centimeter and a melt flow ratio in the range of about 22 to about 70, with the provisos that:
(a) the mixture of ethylene copolymer matrix and active catalyst formed in the first reactor in the series is transferred to the second reactor in the series; PA1 (b) other than the active catalyst referred to in proviso (a) and the cocatalyst referred to in proviso (e), no additional catalyst is introduced into the second reactor; PA1 (c) in the high melt index reactor: PA1 (d) in the low melt index reactor: PA1 (e) additional hydrocarbyl aluminum cocatalyst is introduced into the second reactor in an amount sufficient to restore the level of activity of the catalyst transferred from the first reactor to about the initial level of activity in the first reactor. PA1 (A) increasing or decreasing the melt flow ratio and/or molecular weight of the blend by, respectively, decreasing or increasing the mole ratio of a precursor activator compound to the electron donor; and/or PA1 (B) increasing or decreasing the bulk density of the blend by, respectively, increasing or decreasing the molar ratio of a precursor activator compound to the electron donor PA1 (I) the mole ratio of the precursor activator compound to the electron donor is in the range of about 0.1:1 to about 1:1; PA1 (II) the precursor activator compound can be one compound or a sequential mixture of two different compounds; PA1 (III) each precursor activator compound has the formula M(R.sub.n)X.sub.(3-n) wherein M is Al or B; each X is independently chlorine, bromine, or iodine; each R is independently a saturated aliphatic hydrocarbon radical having 1 to 14 carbon atoms, provided that when M is Al, n is 1 to 3 and when M is B, n is 0 to 1.5; PA1 (IV) the activation of the precursor is carried out prior to the introduction of the precursor into the reactor; and PA1 (V) the activation in proviso(IV) is partial. PA1 (A) increasing or decreasing the melt flow ratio and/or molecular weight of the blend by, respectively, increasing or decreasing the molar ratio of a second precursor activator compound to a first precursor activator compound PA1 (I) the mole ratio of the second precursor activator compound to first precursor activator compound is in the range of about 1:1 to about 6:1; PA1 (II) the two precursor activator compounds are a sequential mixture wherein the first precursor activator compound is the first in the sequence and the second precursor activator compound is the second in the sequence; PA1 (III) each precursor activator compound has the formula M(R.sub.n)X.sub.(3-n) wherein M is Al or B; each X is independently chlorine, bromine, or iodine; each R is independently a saturated aliphatic hydrocarbon radical having 1 to 14 carbon atoms; and n is 1 to 3; PA1 (IV) the activation of the precursor is carried out prior to the introduction of the precursor into the reactor; and PA1 (V) the activation in proviso(IV) is partial.
(1) the alpha-olefin is present in a ratio of about 0.01 to about 3.5 moles of alpha-olefin per mole of ethylene; and PA2 (2) hydrogen is present in a ratio of about 0.05 to about 3 moles of hydrogen per mole of combined ethylene and alpha-olefin; PA2 (1) the alpha-olefin is present in a ratio of about 0.02 to about 3.5 moles of alpha-olefin per mole of ethylene; and PA2 (2) hydrogen is, optionally, present in a ratio of about 0.0001 to about 0.5 mole of hydrogen per mole of combined ethylene and alpha-olefin; and
While the in situ blends prepared as above and the films produced therefrom are found to have the advantageous characteristics heretofore mentioned, there is a desire to fine tune certain properties without making major changes in polymerization conditions. This is of particular importance when one considers the fact that in an in situ blend system such as described, typically two independent reaction systems are linked. Thus, changes in the first reactor propagate to the second reactor over a prolonged time period making control of product properties technically difficult. In addition, in commercial operations, the absolute raw material purity can fluctuate from time to time, causing changes in polymer properties. The invention allows one to more rapidly respond to these changes without prolonged waiting periods for changes in reaction conditions to take effect in both reactors.